Overload Sizing and FLA when using RPC
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  1. #1
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    Default Overload Sizing and FLA when using RPC

    This post is about a RCP that uses a 7.5HP idler and drives a 5HP lathe motor. All motors are wired for 230V.

    I used the FitchW method to balance phases and got these results which are textbook perfect according to the method:

    Idler only:
    Vab = 243
    Vac = 262
    Vbc = 262

    My lathe came equipped with a Furnas reversing contactor setup and solid state overload relay (Furnas 48ASF3M20).

    The problem is that the overload trips during lathe motor startup. The lathe (Monarch 16CY) has a clutch so the motor starts unloaded other than spinning up its own mass and the belt drive system. It only trips on the 2nd and subsequent starts. If it rests a few minutes, it will start without tripping. It also trips if all capacitors are removed but it will start 3-4 times in succession before tripping. Once started, the clutch can be disengaged and the motor has no problem driving the lathe. The overloads have never tripped during operation once the motor made it past startup.

    Can one reliably go by the nameplate data and overload relay setpoint when using a RCP? Is if normal for a motor to operate above its FLA when running on a RCP? Any other troubleshooting suggestions?

    EDIT:
    When the 5HP is running with the clutch engaged (lathe is not turning):
    Vab = 242 V
    Vac = 238 V
    Vbc = 236 V
    Ia = 6.3 A
    Ib = 6.7 A
    Ic = 4.2 A

    With the lathe spinning at 600RPM:
    Vab = 242 V
    Vac = 226 V
    Vbc = 239 V
    Ia = 8.3 A
    Ib = 10.2 A
    Ic = 4.7 A

    My meter doesn't have a max-hold feature but I saw current as high as 92 Amps during startup of the 5HP motor during startup.

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    Those numbers are not textbook perfect. What counts are the numbers when all motors are running steady state.

    The overload relay has adjustment from 13A to 27A. What is your adjustment thumb wheel set to?

    You should have a overload relay on you 7.5Hp idler.

    The Fitch stuff on the web is a beetle-bomb introduction to the RPC world.

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    Did this lathe come from a facility with true 3 phase power? What might be happening is that the OL is sensing an unbalance which would be normal during starting. The relay does have a memory feature that If you try to restart right away, the OL thinks the motor is "hot" and needs a cool down period. Unfortunately, Furnas is out of business, bought out by Siemens, and I doubt that information other than what's on the net is available.

    Tom

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    I tried adjustments on the relay from 14.5A (nameplate FLA) to 18A. Results didn't change. It tripped on the 2nd and subsequent restarts.

    I do not have an overload relay on the 7.5HP. The purpose of this is to protect in the case of a locked rotor or bad bearing I presume?

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    5HP 230V 3 phase, your motor FLA should be around 16.7A. Whatever it says on the motor nameplate is what you must set the OL relay for if you want to protect that motor. If you turn it up, you run the risk of damaging the winding insulation; it's not a negotiation, its a fact. CAN you turn it up? Sure, at your risk of having to replace the motor.

    The ESP100 SSOL relay did have phase current imbalance protection, IIRC it will bias the trip point and the amount of biasing increases with unbalance percentage, meaning the more unbalanced the current is, the lower it will change the trip point in order to protect the motor from overheating due to negative sequence currents that are created by an imbalance. So from the numbers you posted, you have a 39% imbalance when the lathe is spinning, that's a serious issue. And yes the OLR has a thermal memory, so yes, successive starts will cause it to trip, because successive starts are heating up your motor! So that relay is doing its job in both counts.

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    Quote Originally Posted by TDegenhart View Post
    Did this lathe come from a facility with true 3 phase power? What might be happening is that the OL is sensing an unbalance which would be normal during starting. The relay does have a memory feature that If you try to restart right away, the OL thinks the motor is "hot" and needs a cool down period. Unfortunately, Furnas is out of business, bought out by Siemens, and I doubt that information other than what's on the net is available.

    Tom
    You may be on to something with the memory feature. That would explain why it always starts the first time after the RCP is turned on. I'll test if the lathe will start in quick (30s) succession if I remove power from the OL relay between starts to clear the memory (based on a data-free assumption that the relay has no persistent memory).

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    Keep in mind that one of the goals of electronic OL relays is simulate a hot motor. This is simple with thermal relays because the OL gets hot just like the motor and takes time to cool down. With electronic relays this is done with RC capacitive decay. The other benefit of electronics is they can be designed to detect phase unbalance as can bimetal OL's. Solder pots by their vary nature cannot. Remember that OL's of all types are but simulations of true motor conditions and cannot provide perfect protection.

    In the case at hand, it is unlikely that the motor will be called on the provide full power of long periods of time as in production based on your profile. If the electronic relay continues to cause starting problems, change it out to a solder pot or a non differential bimetal relay.

    Tom

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    Quote Originally Posted by *D'B=6bk View Post
    I tried adjustments on the relay from 14.5A (nameplate FLA) to 18A. Results didn't change. It tripped on the 2nd and subsequent restarts.

    I do not have an overload relay on the 7.5HP. The purpose of this is to protect in the case of a locked rotor or bad bearing I presume?
    A overload relay on the idler is going to function same as the overload for the lathe motor. I have three of those ESP100 overload relays. I like them because they are solid state and have a wide adjustment range. I use one on a RPC idler that is not perfectly balanced. And when the target motor is not running it is more out of balance. The overload does not trip because I run the generated leg back through the 3rd connection points on the overload relay. I set my overload relay to 2% higher than the idler motor FLA. But it cannot be that exact because you are working with a little
    thumbscrew that has printing on it.

    Voltages with this relay when target motor is on:
    248.5
    251
    249.5

    A couple of times I was not getting the correct operation. Found one or two loose connections. Once everything was secure the problem was gone.
    Check and clean all wire screw connections.

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    Below is a copy of the time-current curve for the overload relay. TDegenhart was correct in that this relay has a hot and a cold curve and likely the performance of my RCP and lathe motor intersects with that asterisk on the hot curve. FWIW the text associated with the asterisk is "*Hot trip times will vary depending on previous running condition, duty cycle, and length of 'OFF' time"

    esp100-curve.jpg

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    Before you start the lathe again after a second start, try pushing in the white reset button. Does that have any affect on the electronic circuit inside.
    Won't fix anything but it will indicate that the button puts the overload back into a cold state again.

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    No, pushing the reset button would not have any effect on the thermal memory of the ESP100, or any other OL relay for that matter, it's a requirement for UL listing of the OL relay that is does not. Even removing power does not change that on an ESP100 (it unfortunately does on some other brands of SSOL, which is a problem for them).

    Any SSOL will have a fixed cool-down rate that is based on the level of trip current and the motor thermal state (hot vs cold) using specific calculations set up in NEMA design specs for motors. As mentioned, older electro-mechanical OLs had that "baked in" to the design and selection of the bi-metal or melting alloy heater elements, so SSOLs had to emulate that.

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    Then there would be some RC time constant delay of some sort. I have one of those OL relays that you just run the three wires through three holes. The electronics inside are only powered by induction. The one I have in hand shows no way to disassemble it. Would have to destroy the case to see what's inside. Not worth it.

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    Wink

    Inside, all of the electronics are potted in black epoxy anyway. You can't see anything useful.

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    Quote Originally Posted by *D'B=6bk View Post
    Below is a copy of the time-current curve for the overload relay. TDegenhart was correct in that this relay has a hot and a cold curve and likely the performance of my RCP and lathe motor intersects with that asterisk on the hot curve. FWIW the text associated with the asterisk is "*Hot trip times will vary depending on previous running condition, duty cycle, and length of 'OFF' time"

    esp100-curve.jpg
    It is a righteous-enough protective device for "native", balanced, 3-P power.

    As Tom said, it is also the wrong device for the PRESENT circumstances.

    Just swap it out for something brute-dumb, or at least less "conceited'.. before the "usual suspects" among KTE's and armchair theorizers swamp all common sense, re-engineer it or breadboard a modified version instead of shedding it, and you never get to make another chip!



    Starting load is wot it is. Brutal. Also brief. So what?

    Your motor is mature enough to very nearly ignore the minor issue of a "generated" leg being less than perfect. It loses about ten percent of its comfortable max power, where "comfort" includes even distribution of heating effects at or near max load over LONG, not short, periods of hard running. Also less than 100% perfection as to torque and HP, "all the time".

    Bee Eff Dee. Mostly neither of those is noticed at all 'coz you are demanding less than HALF of "nameplate" for ordinary turning.

    Concerned about that? Pocket-change and you can implement a fair-decent "load meter" within peripheral vision. Simply giving a damn about the longevity of your investment so as to not abuse it does the rest.

    Try it. Ignorant "shunt" type AC Ammeter, one leg. It could be interesting, even useful, as well as cheap.

    Fair bet most work is down in the 20% to 40% load range, if-even that much.

    Most of the work we Chick'ns present to these industrial-grade warhorses is only a minor annoyance to their generous 24 X 6 X 365 three shift, Sunday's off war-work load budgets.

    It's a five hoss Monarch 16CY, not a half-hoss South Bend 9".

    What is the difference between a horse and a pony?

    2CW

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    Quote Originally Posted by Jraef View Post
    Inside, all of the electronics are potted in black epoxy anyway. You can't see anything useful.
    Not this time. I found a little screw that holds a panel.

    dsc_1024.jpg

    A custom IC, three capacitors, two potentiometers , a relay, and three coils. The IC is printed with "Furnas D81649-1D".
    To the left are two black rubber boots that each cover a set of wire-wrap posts. The pick-up coil wires are wire-wrapped to the board.
    The relay to the right is also connected by wire-wrap posts. This thing is serviceable.

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    Quote Originally Posted by *D'B=6bk View Post
    This post is about a RCP that uses a 7.5HP idler and drives a 5HP lathe motor. All motors are wired for 230V.

    With the lathe spinning at 600RPM:
    Vab = 242 V
    Vac = 226 V
    Vbc = 239 V
    Ia = 8.3 A
    Ib = 10.2 A
    Ic = 4.7 A
    My suggestion is to adjust the balance with the 7.5Hp with lathe spinning. With the numbers I measured in post #8 the no-load Vac and Vbc was around 282.
    I suspect that the overload is sensing enough phase imbalance to trip the relay.
    Those numbers are not good and you can get them much better with some capacitor shuffling.

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    Quote Originally Posted by rons View Post
    Those numbers are not good and you can get them much better with some capacitor shuffling.
    Nonsense. The numbers aren't that bad, and the lathe was running unloaded, not in a cut of any kind, let alone light or heavy.

    Tuning of caps on RPC's is load-specific. Waste of time to overdo it.

    The protective device has to be less sensitive. "Period".

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    Quote Originally Posted by *D'B=6bk View Post
    It only trips on the 2nd and subsequent starts.

    My meter doesn't have a max-hold feature but I saw current as high as 92 Amps during startup of the 5HP motor during startup.
    The first time is when the overload circuit is not powered from time zero. Surge is not detected or acted upon. The surge in current is not equal across all incoming phases.

    The second time and after is detecting the surge imbalance because the circuit is energized and working from time zero.

    Balance it or pay the price. The price is that over time your 5Hp motor will heat above it's normal operating.

    I have 3 phase motor that trips over phase unbalance current ,so the question is where does this unbalance current goes in the motor ?

    http://www.cooperindustries.com/cont..._Unbalance.pdf

    Electrical Unbalance Kills Motors - Efficient Plant

    Monarch butterflies are sensitive beings that fall victim to weather extremes, from HIGH HEAT and intense cold to strong storms to drought.

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    Some new info here from OP:

    I tried waiting 20 minutes between restarts and it tripped. So I tried waiting an hour, still tripped. It seems that the aptly-named ESP100 senses when I've given up and moved onto some other project before returning to a cold-start state.

    In addition, a local electrical supply house is liquidating and I have at my disposal virtually any overload device made in the last 20 years for pennies on the dollar. Any specific recommendations for a overload relay that won't mind the phase imbalance?

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    Quote Originally Posted by *D'B=6bk View Post
    Some new info here from OP:

    I tried waiting 20 minutes between restarts and it tripped. So I tried waiting an hour, still tripped. It seems that the aptly-named ESP100 senses when I've given up and moved onto some other project before returning to a cold-start state.

    In addition, a local electrical supply house is liquidating and I have at my disposal virtually any overload device made in the last 20 years for pennies on the dollar. Any specific recommendations for a overload relay that won't mind the phase imbalance?
    Old skewl "heaters" that only covered two legs of the three in the first place let you skip the generated leg as would not ordinarily be "the one" drawing enough current to be first to trip, regardless.

    "We presume" that you don't saddle this partic'lar sow-hog for naked midnight rides, "lights out" and would "be there" to shut it OFF by hand if it locked-up on startup, or if anything begin to stank of fecal matter or the like whilst in the cut, yah?

    What have we missed as could actually go wrong, anyway?

    Ah well. The process of gold-plating a turd starts with dehydration, followed by electroless carbon, thence a strike of flash Copper, then....

    N'er mind. It's still a turd, shiney or never.

    Or perhaps just a hard-working over-age-in-grade Junior Angel in steel toads and grubby jeans, in this case?

    "Run what you got." It beats all Hell out of "Getting the runs". DAMHIKT, either!



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